In the following specification some numerical values are reported wherein the dot “.” is used as separator between the integer and fractional parts of the value. Moreover, in the following specification and in the claims, where the type of connection is not expressly specified, the terms “connection” and “connected” (and their variations) should be intended as referring to a connection consisting in either a direct short-circuit connection or a connection through an interposed component (e.g. a voltage or current measuring component).
It is known that, along with the standard error correction techniques based on the fundamental concepts of the classical negative feed-back (sometimes simply called feed-back), widely used in both signal and power amplifiers, other supplementary error correction techniques have been developed during the last years, which are often synergically used in amplifier circuits with the most traditional ones for improving their distortion performance, and which can be basically grouped in four types.
The first type of supplementary error correction technique comprises the correction techniques internal to the main negative feedback loop, also known as “nested differentiating feed-back loops”, e.g. disclosed by Edward M. Cherry in “A new result in negative feedback theory and its application to audio power amplifiers”, Int. Journal Circuit Theory, Vol. 6, pp. 265-288, July 1978, in “A Power Amplifier Improver”, J. Audio Eng. Soc., Vol 29, No. 3, pp. 140-147, March 1981, and in “Nested differentiating feedback loops in simple power amplifiers”, J. Audio Eng. Soc., V. 30, pp. 295-305, May 1982, as well as in the U.S. Pat. No. 4,243,943 to Cherry. In such first type of technique, one or more special negative feedback loops are inserted along the signal path, which are characterized by pairs of poles-zeros inserted in proper position so as to increase the gain available at the highest frequencies without excessively altering the stability margins of the whole amplifier. Among the practical limits of such scheme, besides difficulties associated with the choice of the poles-zeros pairs and with their practical implementation, it should be considered that generally it includes the insertion, for each correcting loop, of some gain and/or support blocks along the signal path, which interfere with the response of the amplifier to correct, on the one hand, and which are in turn non negligible sources of further errors, on the other hand. Therefore, practically obtainable improvements are limited and such first type of supplementary error correction technique is not used in audio amplifiers with requirements of very low distortion limited to few parts per million in the whole audio frequency band.
The second type of supplementary error correction technique comprises the supplementary negative feedback techniques which add to the main loop, known as techniques of “active-error feedback”, e.g. disclosed by W. Baggally in “Distortion Cancellation in Audio Amplifiers”, Wireless Eng. & Experim. Wireless, Vol. 10, pp. 413-419, August 1933, by J. R. Macdonald in “Active-Error Feedback and its Application to a Specific Driver Circuit”, Proc. IRE, vol. 43, pp. 808-813, July 1955, by D. Bollen in “Distortion Reducer”, Wireless World, February 1973, pp. 54-57, and by U.S. Pat. No. 6,275,102 to Muza. In such techniques a supplementary error negative feedback correction loop is added, wherein the correction voltage is directly added to the input voltage with no need for insertion of additional blocks along the main signal path, thus being less invasive of the techniques of the first type. However, due to the high interactions that in such scheme the correction loop has intrinsically with the main useful signal path, it is practically impossible to exploit in the best way such potentialities and at the same to keep satisfactory dynamic stability margins of the corrected amplifier in every operative condition, and hence more in general a good “time-frequency response” of the corrected amplifier (in the present specification and in the claims, it is jointly and synthetically meant with “time-frequency response” the response in the time domain along with the one in the frequency domain). The frequency range wherein the active negative feedback loop may operate with adequate dynamic stability margins of the whole amplifier must be necessarily reduced in general to a modest fraction of the band wherein the correction loop could instead operate with good stability margins, consequently limiting the use of such techniques to applications with only one correction loop in audio amplifiers having few stages and in servo systems operating at low frequencies.
The third type of supplementary error correction technique comprises the feedback error-correction techniques, wherein the error is measured and extracted with high accuracy from the output and added with the same accuracy and with proper phase to the input signal, e.g. as disclosed by Malcom J. Hawksford in “Distortion Correction In Audio Power Amplifiers”, J. Audio Engineering Society, Vol. 29, No. 1/2, 1981 January/February, and in “Power amplifier output stage design incorporating error feedback correction with current damping enhancement” presented at the 74th AES Convention, 1983, October 8-12, New York, by Robert R. Cordell in “A Mosfet Power Amplifier with Error Correction”, J. Audio Eng. Soc., Vol 32, No. 1/2, 1984 January/February, and in the U.S. Pat. Nos. 5,892,398 and 6,600,367 to Candy. These techniques can be considered as a special application of the active-error feedback ones of the second type, with which they substantially share the same dynamic stability problems and mentioned applicability limits. Such techniques, though effective in principle for reducing distortion, are not very spread both because they are complex from a circuit and design point of view, and because of the unavoidable and expensive criticalities associated with them, since they require in general the use of critical circuits calibrated in a wide frequency band for accurately performing the error extraction and sum, which in turn are unfortunately sources of further errors, which, for the same nature of the correction mechanism, are not correctable by the same negative feedback loop introduced for correcting the error of the output stage.
The fourth type of supplementary error correction technique comprises the error feed-forward techniques, e.g. disclosed in the U.S. Pat. No. 1,686,792 to Black, by Vanderkooy J. and Lipshitz S. P. in “Feed-forward error correction in power amplifiers”, J. Audio Eng. Soc., V. 28, pp. 2-16, January/February 1980, and in the Patent Application EP 1913687 and in the U.S. Pat. No. 7,564,304 to Stochino et al. Among these techniques, there are canonical techniques (also known as true error feed-forward), wherein the error correction loop is always outside possible negative feedback loops and hence which do not suffer from typical problems of dynamic stability from which the latter suffer, and the combined techniques (feed-back and feed-forward), wherein the feed-forward error correction is employed inside the general feedback loop, and that hence does not enjoy the unconditioned stability benefits which are instead enjoyed by the canonical feed-forward correction loops. The canonical techniques of pure feed-forward correction are based on mechanisms capable to accurately extract and isolate the error voltage of the amplifier to correct and to add it in anti-phase to the output signal, so as to virtually cancel its error. However, although much used in the radio-microwave field, such techniques suffer from some implementation problems, related to the need for delicate and expensive calibrations and to the presence of a transformer adder with magnetic flux cancellation that has limited and expensive application. Also, practical applications of such techniques are limited to power amplifiers with discrete components, and such techniques barely find convenient applications in the integrated circuit field.